Internal combustion engine exhaust contains environmentally and biologically harmful compositions, including hydrocarbons, carbon monoxide, and nitrogen oxide which arise from combustion of gasoline or other fuels. Catalytic converters were developed for vehicles to reduce the emission of these harmful compositions, and, in the U.S., have been installed on passenger cars and light-duty trucks since 1975.
Catalytic converters reduce vehicle exhaust emission levels by chemically converting engine-out emissions before the exhaust gas leaves the tailpipe. Conventionally, a catalytic converter contains a “honeycomb” ceramic substrate housed in a stainless steel canister that directs exhaust gases through narrow channels. A catalyst layer is applied to the surface of the channels and facilitates the conversion of pollutants primarily into water vapor, carbon dioxide, and nitrogen. The catalysts employed in most cases are noble metals, such as platinum (Pt), rhodium (Rh), and palladium (Pd).
Current catalytic converters are commonly referred to as three-way catalytic converters due to the three simultaneous reactions occurring over the catalyst. These include two oxidation reactions to reduce hydrocarbons (HC) and carbon monoxide (CO) and a reduction reaction involving oxides of nitrogen (NOx) with CO over a suitable catalyst to reduce NOx to nitrogen gas and carbon dioxide.
The exact combination of catalytic metals differs according to the level of engine-out emissions and the required emission reductions. Current catalytic converter designs are more than 95% efficient in removing HCs and CO, and at least 85% effective at reducing NOx over the lifetime of the converter.
Traditionally, catalytic converters are prepared by separately mixing oxidative precious metals, such as platinum or palladium, with aluminum oxide, water, and other components to make a slurry in one container and mixing one or more reductive precious metals, such as rhodium, with cerium zirconium oxide, water, and other components to make a second slurry in a second container. The slurries are normally referred to as oxidative and reductive washcoats. A ceramic monolith, which can be cylindrically shaped, having a grid or “honeycomb” array structure, is dipped into one of the washcoats to form a first catalytic layer on the monolith. After drying and calcining, the ceramic monolith is dipped into the other washcoat to form a second layer. The ceramic monolith is then fitted into a shell of a catalytic converter, which connects to the engine for treating exhaust gas.